diff --git a/ui-tui/src/__tests__/submissionCore.test.ts b/ui-tui/src/__tests__/submissionCore.test.ts
new file mode 100644
index 000000000..83b89a088
--- /dev/null
+++ b/ui-tui/src/__tests__/submissionCore.test.ts
@@ -0,0 +1,131 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { isSessionBusyError, markSubmitting, submitPrompt, type SubmitPromptDeps } from '../app/submissionCore.js'
+import { getUiState, patchUiState, resetUiState } from '../app/uiStore.js'
+import type { GatewayClient } from '../gatewayClient.js'
+
+// A gateway double whose `input.detect_drop` resolution we control, so we can
+// observe UI state DURING the async gap — the exact window the queue-mode race
+// lived in.
+function makeDeferredGateway() {
+  let resolveDrop: (v: unknown) => void = () => {}
+
+  const dropPromise = new Promise(res => {
+    resolveDrop = res
+  })
+
+  const calls: string[] = []
+
+  const gw = {
+    request: vi.fn((method: string) => {
+      calls.push(method)
+
+      if (method === 'input.detect_drop') {
+        return dropPromise
+      }
+
+      // prompt.submit et al: resolve immediately with a success shape.
+      return Promise.resolve({ status: 'streaming' })
+    })
+  } as unknown as GatewayClient
+
+  return { calls, gw, resolveDrop: (v: unknown = { matched: false }) => resolveDrop(v) }
+}
+
+function makeDeps(gw: GatewayClient, over: Partial<SubmitPromptDeps> = {}): SubmitPromptDeps {
+  return {
+    appendMessage: vi.fn(),
+    enqueue: vi.fn(),
+    expand: (t: string) => t,
+    gw,
+    maybeGoodVibes: vi.fn(),
+    setLastUserMsg: vi.fn(),
+    sys: vi.fn(),
+    ...over
+  }
+}
+
+describe('submissionCore.submitPrompt — synchronous busy (queue-race fix)', () => {
+  beforeEach(() => {
+    resetUiState()
+    patchUiState({ sid: 'sess-1' })
+  })
+
+  it('flips busy=true SYNCHRONOUSLY, before input.detect_drop resolves', () => {
+    const { gw, resolveDrop } = makeDeferredGateway()
+
+    expect(getUiState().busy).toBe(false)
+
+    submitPrompt('hello', makeDeps(gw))
+
+    // The critical invariant: busy is already true even though the
+    // detect_drop RPC has NOT resolved yet. This is what makes a second,
+    // rapid submit take the local-enqueue branch instead of racing a second
+    // prompt.submit onto the backend.
+    expect(getUiState().busy).toBe(true)
+    expect(getUiState().status).toBe('running…')
+
+    resolveDrop()
+  })
+
+  it('regression: two back-to-back sends — the SECOND sees busy=true in the gap', async () => {
+    const { gw, resolveDrop } = makeDeferredGateway()
+
+    // Emulate dispatchSubmission's routing decision: it sends only when
+    // busy===false, otherwise it would enqueue. We assert the state the
+    // router reads, which is the real regression.
+    submitPrompt('first message', makeDeps(gw))
+
+    // Before the fix, busy was still false here (set only inside detect_drop's
+    // .then), so a second Enter would wrongly route into send() again.
+    const busyWhenSecondArrives = getUiState().busy
+    expect(busyWhenSecondArrives).toBe(true)
+
+    resolveDrop()
+    await Promise.resolve()
+  })
+
+  it('does not submit when there is no session, and does not mark busy', () => {
+    resetUiState() // sid: null
+    const { gw, calls } = makeDeferredGateway()
+    const sys = vi.fn()
+
+    submitPrompt('hello', makeDeps(gw, { sys }))
+
+    expect(getUiState().busy).toBe(false)
+    expect(sys).toHaveBeenCalledWith('session not ready yet')
+    expect(calls).not.toContain('input.detect_drop')
+  })
+
+  it('after detect_drop resolves (no file), it issues prompt.submit', async () => {
+    const { calls, gw, resolveDrop } = makeDeferredGateway()
+
+    submitPrompt('hi there', makeDeps(gw))
+    expect(calls).toEqual(['input.detect_drop'])
+
+    resolveDrop({ matched: false })
+    await Promise.resolve()
+    await Promise.resolve()
+
+    expect(calls).toContain('prompt.submit')
+  })
+})
+
+describe('submissionCore.markSubmitting', () => {
+  beforeEach(() => resetUiState())
+
+  it('sets busy + running status', () => {
+    markSubmitting()
+    expect(getUiState().busy).toBe(true)
+    expect(getUiState().status).toBe('running…')
+  })
+})
+
+describe('submissionCore.isSessionBusyError', () => {
+  it('matches the legacy busy rejections but not arbitrary errors', () => {
+    expect(isSessionBusyError(new Error('session busy'))).toBe(true)
+    expect(isSessionBusyError(new Error('waiting for model response'))).toBe(true)
+    expect(isSessionBusyError(new Error('some other failure'))).toBe(false)
+    expect(isSessionBusyError('not an error')).toBe(false)
+  })
+})
diff --git a/ui-tui/src/app/submissionCore.ts b/ui-tui/src/app/submissionCore.ts
new file mode 100644
index 000000000..7c561b745
--- /dev/null
+++ b/ui-tui/src/app/submissionCore.ts
@@ -0,0 +1,111 @@
+import { attachedImageNotice } from '../domain/messages.js'
+import type { GatewayClient } from '../gatewayClient.js'
+import type { InputDetectDropResponse, PromptSubmitResponse } from '../gatewayTypes.js'
+import type { Msg } from '../types.js'
+
+import { turnController } from './turnController.js'
+import { getUiState, patchUiState } from './uiStore.js'
+
+const SESSION_BUSY_RE = /session busy|waiting for model response/i
+
+export const isSessionBusyError = (e: unknown) => e instanceof Error && SESSION_BUSY_RE.test(e.message)
+
+export interface SubmitPromptDeps {
+  appendMessage: (msg: Msg) => void
+  enqueue: (text: string) => void
+  expand: (text: string) => string
+  gw: GatewayClient
+  maybeGoodVibes: (text: string) => void
+  setLastUserMsg: (value: string) => void
+  sys: (text: string) => void
+}
+
+// Optimistically flip the session to busy the INSTANT a prompt is accepted for
+// submission — synchronously, before we await anything.
+//
+// This is the fix for the queue-mode race (display.busy_input_mode: queue):
+// the submit path first fires an async `input.detect_drop` RPC and only marked
+// the session busy inside that RPC's `.then`. A second Enter pressed inside
+// that round-trip window read `busy === false` in dispatchSubmission and raced
+// a second `prompt.submit` onto the backend instead of landing in the local
+// queue. That produced the reported symptom: the second message "waited for
+// the first to respond, then went to the queue", and the client lost track of
+// it (the backend accepts a mid-turn submit as {status:"queued"} — a success,
+// not an error — so the local drain effect that watches the client-side queue
+// never fires, leaving the UI stuck on "analyzing…" until Ctrl+C).
+//
+// Marking busy at the choke point closes the gap for every caller: the mainline
+// submit, queue-edit picks, and the drain effect all funnel through here.
+export function markSubmitting(): void {
+  patchUiState({ busy: true, status: 'running…' })
+}
+
+// Submit a ready prompt (already resolved to be neither a slash command nor a
+// shell escape, with a live session). Pulled out of useSubmission so the
+// synchronous-busy invariant above is unit-testable without React test infra.
+export function submitPrompt(text: string, deps: SubmitPromptDeps, showUserMessage = true): void {
+  const sid = getUiState().sid
+
+  if (!sid) {
+    return deps.sys('session not ready yet')
+  }
+
+  // Close the async-busy gap up front, before the detect_drop round-trip.
+  markSubmitting()
+
+  const startSubmit = (displayText: string, submitText: string, show = true) => {
+    const liveSid = getUiState().sid
+
+    if (!liveSid) {
+      return deps.sys('session not ready yet')
+    }
+
+    turnController.clearStatusTimer()
+    deps.maybeGoodVibes(submitText)
+    deps.setLastUserMsg(text)
+
+    if (show) {
+      deps.appendMessage({ role: 'user', text: displayText })
+    }
+
+    patchUiState({ busy: true, status: 'running…' })
+    turnController.bufRef = ''
+    turnController.interrupted = false
+
+    deps.gw.request<PromptSubmitResponse>('prompt.submit', { session_id: liveSid, text: submitText }).catch((e: Error) => {
+      // Defensive: prompt.submit no longer rejects a mid-turn send with
+      // "session busy" (the gateway queues it and returns success), but keep
+      // the re-queue path as a safety net for any future/legacy gateway that
+      // still errors, so a message is never silently dropped.
+      if (isSessionBusyError(e)) {
+        deps.enqueue(submitText)
+        patchUiState({ busy: true, status: 'queued for next turn' })
+
+        return deps.sys(`queued: "${submitText.slice(0, 50)}${submitText.length > 50 ? '…' : ''}"`)
+      }
+
+      deps.sys(`error: ${e.message}`)
+      patchUiState({ busy: false, status: 'ready' })
+    })
+  }
+
+  // Always ask the backend whether this looks like a file drop. The backend's
+  // _detect_file_drop handles paths with spaces, quotes, Windows drive letters,
+  // and escaped characters correctly.
+  deps.gw
+    .request<InputDetectDropResponse>('input.detect_drop', { session_id: sid, text })
+    .then(r => {
+      if (!r?.matched) {
+        return startSubmit(text, deps.expand(text), showUserMessage)
+      }
+
+      if (r.is_image) {
+        turnController.pushActivity(attachedImageNotice(r))
+      } else {
+        turnController.pushActivity(`detected file: ${r.name}`)
+      }
+
+      startSubmit(r.text || text, deps.expand(r.text || text), showUserMessage)
+    })
+    .catch(() => startSubmit(text, deps.expand(text), showUserMessage))
+}
diff --git a/ui-tui/src/app/useSubmission.ts b/ui-tui/src/app/useSubmission.ts
index a72f835c9..6ece0bf64 100644
--- a/ui-tui/src/app/useSubmission.ts
+++ b/ui-tui/src/app/useSubmission.ts
@@ -1,28 +1,20 @@
 import { type MutableRefObject, useCallback, useEffect, useRef } from 'react'
 
 import { TYPING_IDLE_MS } from '../config/timing.js'
-import { attachedImageNotice } from '../domain/messages.js'
 import { completionToApplyOnSubmit, looksLikeSlashCommand } from '../domain/slash.js'
 import type { GatewayClient } from '../gatewayClient.js'
-import type {
-  InputDetectDropResponse,
-  PromptSubmitResponse,
-  SessionSteerResponse,
-  ShellExecResponse
-} from '../gatewayTypes.js'
+import type { SessionSteerResponse, ShellExecResponse } from '../gatewayTypes.js'
 import { asRpcResult } from '../lib/rpc.js'
 import { hasInterpolation, INTERPOLATION_RE } from '../protocol/interpolation.js'
 import { PASTE_SNIPPET_RE } from '../protocol/paste.js'
 import type { Msg } from '../types.js'
 
 import type { ComposerActions, ComposerRefs, ComposerState, PasteSnippet } from './interfaces.js'
+import { submitPrompt } from './submissionCore.js'
 import { turnController } from './turnController.js'
 import { getUiState, patchUiState } from './uiStore.js'
 
 const DOUBLE_ENTER_MS = 450
-const SESSION_BUSY_RE = /session busy|waiting for model response/i
-
-const isSessionBusyError = (e: unknown) => e instanceof Error && SESSION_BUSY_RE.test(e.message)
 
 const expandSnips = (snips: PasteSnippet[]) => {
   const byLabel = new Map<string, string[]>()
@@ -88,62 +80,19 @@ export function useSubmission(opts: UseSubmissionOptions) {
     (text: string, showUserMessage = true) => {
       const expand = expandSnips(composerState.pasteSnips)
 
-      const startSubmit = (displayText: string, submitText: string, showUserMessage = true) => {
-        const sid = getUiState().sid
-
-        if (!sid) {
-          return sys('session not ready yet')
-        }
-
-        turnController.clearStatusTimer()
-        maybeGoodVibes(submitText)
-        setLastUserMsg(text)
-
-        if (showUserMessage) {
-          appendMessage({ role: 'user', text: displayText })
-        }
-
-        patchUiState({ busy: true, status: 'running…' })
-        turnController.bufRef = ''
-        turnController.interrupted = false
-
-        gw.request<PromptSubmitResponse>('prompt.submit', { session_id: sid, text: submitText }).catch((e: Error) => {
-          if (isSessionBusyError(e)) {
-            composerActions.enqueue(submitText)
-            patchUiState({ busy: true, status: 'queued for next turn' })
-
-            return sys(`queued: "${submitText.slice(0, 50)}${submitText.length > 50 ? '…' : ''}"`)
-          }
-
-          sys(`error: ${e.message}`)
-          patchUiState({ busy: false, status: 'ready' })
-        })
-      }
-
-      const sid = getUiState().sid
-
-      if (!sid) {
-        return sys('session not ready yet')
-      }
-
-      // Always ask the backend whether this looks like a file drop.
-      // The backend's _detect_file_drop handles paths with spaces, quotes,
-      // Windows drive letters, and escaped characters correctly.
-      gw.request<InputDetectDropResponse>('input.detect_drop', { session_id: sid, text })
-        .then(r => {
-          if (!r?.matched) {
-            return startSubmit(text, expand(text), showUserMessage)
-          }
-
-          if (r.is_image) {
-            turnController.pushActivity(attachedImageNotice(r))
-          } else {
-            turnController.pushActivity(`detected file: ${r.name}`)
-          }
-
-          startSubmit(r.text || text, expand(r.text || text), showUserMessage)
-        })
-        .catch(() => startSubmit(text, expand(text), showUserMessage))
+      submitPrompt(
+        text,
+        {
+          appendMessage,
+          enqueue: composerActions.enqueue,
+          expand,
+          gw,
+          maybeGoodVibes,
+          setLastUserMsg,
+          sys
+        },
+        showUserMessage
+      )
     },
     [appendMessage, composerActions, composerState.pasteSnips, gw, maybeGoodVibes, setLastUserMsg, sys]
   )